1. Field of the Invention
The present invention relates to a sheet carrying device and an image forming apparatus including the sheet carrying device.
2. Description of the Related Art
In a commercial print industry, small lot printing, many print types, variable data printing, or the like are becoming more frequently performed by a document printer using an electrophotographic method, which is called Print On Demand (POD), than by a conventional offset printer. In order to deal with various needs such as those described above, front and back registering accuracy, image evenness, or the like, comparable to that of an offset printer is required for the electrophotographic document printers.
Factors causing a front and back register gap in the electrophotographic document printer are registration error in the longitudinal and lateral directions, skew error between a sheet and a print image, and image expansion and contraction on a sheet caused in a toner transferring unit. Image expansion and contraction on the sheet caused in the toner transferring unit disturbs image evenness to be one of factors causing an abnormal image such as banding.
Image expansion and contraction in the toner transferring unit is caused by incorrect transfer of a toner image formed on a photoreceptor drum, an intermediate transferring belt, or the like depending on variation of the sheet carrying speed, at which an image is transferred in the toner transferring unit.
FIG. 10 illustrates a schematic structure of an image forming apparatus 110 of the related art. The image forming apparatus 110 includes a photoreceptor drum 13, a transferring roller 14, a fixing unit 50, or the like. A toner image is formed on a surface of a sheet S.
The sheet S is interposed between a driving roller 12 which is rotated by a motor 31 and a driving mechanism and a driven roller 11 which is rotated by contacting the driving roller 12 so as to be carried. The sheet S is further carried between the photoreceptor drum 13 which is rotated by a motor 32 and a transferring roller 14 which is rotated by a motor 33. A toner image formed on the surface of the photoreceptor drum 13 is transferred to the sheet S in a transferring portion between the photoreceptor drum 13 and the transferring roller 14. Simultaneously, the sheet S is further interposed between the photoreceptor drum 13 and the transferring roller 14 so as to be carried to a fixing unit 50 by a carrying belt 23. The toner image is fixed by applying heat and pressure to the surface of the sheet S, which is carried by the fixing unit 50, at a time of passing through a space between a heat roller 51 and a pressure roller 52. The sheet S having the toner image fixed to it is ejected out of the electrophotopgraphic document printer.
At the time of transferring the toner image, the sheet S is interposed between the driven roller 11 and the driving roller 12 and also between the photoreceptor drum 13 and the transferring roller 14 so as to be carried, or interposed between the photoreceptor drum 13 and the transferring roller 14 so as to be carried. Therefore, the carrying speed of the sheet S is determined by the peripheral speeds of the driving roller 12, the photoreceptor drum 13, and the transferring roller 14.
If the carrying speed of the sheet S varies while the toner image is being transferred, an abnormal image such as banding may be produced as described above. Therefore, it is necessary to maintain the carrying speed evenly by minutely adjusting each of the peripheral speeds of the driving roller 12, the photoreceptor drum 13, and the transferring roller 14. However, a friction coefficient on the surface of the photoreceptor drum 13 may be changed by a toner adhesion amount. An influence on the carrying speed of the sheet S varies depending on the change of the friction coefficient, an environmental change of temperature and humidity or the like, a temporal change of various rollers, or the like. Therefore, it is difficult to stabilize the carrying speed of the sheet S by minutely adjusting peripheral speeds of the various portions depending on the above complicated changes.
There is an example where, while the sheet S is interposed between the driven roller 11 and the driving roller 12 and also between the photoreceptor drum 13 and the transferring roller 14 so as to be carried, a torque limiter is provided in a driving mechanism of the driving roller 12 so that the driving roller is driven to rotate by the sheet (for example, Patent Document 1 or 2). When the sheet S is interposed between the driven roller 11 and the driving roller 12 and also between the photoreceptor drum 13 and the transferring roller 14 so as to be carried, the torque limiter is provided to cut off the driving force from the motor 31 to the driving roller 12 by slippage in the torque limiter. Thus, the driving motor 31 is driven to rotate by the sheet S. With this structure, it is possible to reduce the influence of the driving roller 12 on the carrying speed of the sheet S in transferring the toner thereby stabilizing the carrying speed of the sheet S.
There is an example structure where a torque limiter is provided in a driving mechanism of the transferring roller 14 to cause the torque limiter to slip while the transferring roller 14 contacts the photoreceptor drum 13 for cutting off driving force from the motor 33 to the transferring roller 14. In this structure, the transferring roller 14 is driven to rotate by the photoreceptor drum 13 (for example, Patent Document 3). With this structure, an influence of the transferring roller 14 on the carrying speed of the sheet S is reduced. Thus, the variation of sheet carrying speed in transferring a toner image to the sheet S can be reduced.
However, in the above structure of the driving mechanism of the driving roller 12 having the torque limiter, the carrying speed of the sheet S depends on any one of peripheral speeds of the photoreceptor drum 13 and the transferring roller 14. Therefore, there may be a case where it is difficult to carry the sheet S at an accurately uniform speed depending on the toner adhesion amount on the photoreceptor drum 13, environmental change, temporal change, or the like.
Further, in a case where the torque limiter is provided in the driving mechanism of the transferring roller 14, while the sheet S is interposed between the driven roller 11 and the driving roller 12 and also between the photoreceptor drum 13 and the transferring roller 14, there is a probability that the carrying speed of the sheet S is not stabilized by an influence from the driving roller 12.
Patent Document 1: Japanese Laid-Open Patent Application No. 07-140740
Patent Document 2: Japanese Laid-Open Patent Application No. 2005-15217
Patent Document 3: Japanese Laid-Open Patent Application No. 11-52757